CanvasTextWrapper/CanvasTextWrapper.js

/*! CanvasTextWrapper (https://github.com/namniak/CanvasTextWrapper)
 *  Version:  0.1.0
 *
 *  MIT License (http://www.opensource.org/licenses/mit-license.html)
 *  Copyright (c) 2014 Vadim Namniak
 */

(function() {
    'use strict';

    var defaultOptions = {
        font: '18px Arial, sans-serif',
        textAlign: 'left',     // each line of text is aligned left
        verticalAlign: 'top',  // text lines block is aligned top
        paddingX: 0,           // zero px left & right text padding relative to canvas or parent
        paddingY: 0,           // zero px top & bottom text padding relative to canvas or parent
        fitParent: false,      // text is tested to fit canvas width
        lineBreak: 'auto'      // text fills the element's (canvas or parent) width going to a new line on a whole word
    };

    window.CanvasTextWrapper = function(canvas, text, opts) {

        if (!(this instanceof CanvasTextWrapper)) {
            throw new TypeError('CanvasTextWrapper constructor failed. Use "new" keyword when instantiating.');
        }

        this.canvas = canvas;
        this.text = text;

        // set options to specified or default values
        for (var property in defaultOptions) {
            this[property] = (opts && opts[property]) ? opts[property] : defaultOptions[property];
        }

        // extract font size
        this.lineHeight = parseInt(this.font.replace(/^\D+/g, ''), 10);

        // validate all set properties
        this.validate();

        // basic context settings
        this.context = this.canvas.getContext('2d');
        this.context.font = this.font;
        this.context.textBaseline = 'bottom';

        this.drawText();
    };

    CanvasTextWrapper.prototype = {

        drawText: function() {
            var canvas = this.canvas;
            var context = canvas.getContext('2d');

            var elementWidth = (this.fitParent === false) ? canvas.width : canvas.parentNode.clientWidth;
            var maxTextLength = elementWidth - (this.paddingX * 2);

            var words = this.text.split(/\s+/);
            var lines = [];
            var textPos = {
                x: 0,
                y: 0
            };

            this.checkWordsLength(context, words, maxTextLength);
            this.breakTextIntoLines(context, lines, words, maxTextLength);

            // height of the broken down into lines text
            var textBlockHeight = lines.length * this.lineHeight;

            // set vertical align for the whole text block
            this.setTextVerticalAlign(textPos, textBlockHeight);

            for (var i = 0; i < lines.length; i++) {
                this.setTextHorizontalAlign(context, textPos, elementWidth, lines[i]);

                textPos.y = parseInt(textPos.y) + parseInt(this.lineHeight);
                context.fillText(lines[i], textPos.x, textPos.y);
            }
        },

        checkWordsLength: function(context, words, maxTextLength) {
            for (var i = 0; i < words.length; i++) {
                var testString = '';
                var tokenLen = context.measureText(words[i]).width;

                // check if a word exceeds the element's width
                if (tokenLen > maxTextLength) {
                    for (var k = 0; (context.measureText(testString + words[i][k]).width <= maxTextLength) && (k < words[i].length); k++) {
                        testString += words[i][k];
                    }

                    // break the word because it's too  long
                    var sliced = words[i].slice(0, k);
                    var leftover = words[i].slice(k);
                    words.splice(i, 1, sliced, leftover);
                }
            }
        },

        breakTextIntoLines: function(context, lines, words, maxTextLength) {
            for (var i = 0, j = 0; i < words.length; j++) {
                lines[j] = '';

                if (this.lineBreak === 'auto') {
                    // put as many full words in a line as can fit element
                    while ((context.measureText(lines[j] + words[i]).width <= maxTextLength) && (i < words.length)) {
                        lines[j] += words[i] + ' ';
                        i++;
                    }
                    lines[j] = lines[j].trim();
                } else if (this.lineBreak === 'word') {
                    // put each next word in a new line
                    lines[j] = words[i];
                    i++;
                }
            }
        },

        setTextHorizontalAlign: function(context, textPos, elementWidth, line) {
            if (this.textAlign === 'center') {
                textPos.x = (elementWidth - context.measureText(line).width) / 2;
            } else if (this.textAlign === 'right') {
                textPos.x = elementWidth - context.measureText(line).width - this.paddingX;
            } else {
                textPos.x = this.paddingX;
            }
        },

        setTextVerticalAlign: function(textPos, textBlockHeight) {
            var elementHeight = (this.fitParent === false) ? this.canvas.height : this.canvas.parentNode.clientHeight;

            if (this.verticalAlign === 'middle') {
                textPos.y = (elementHeight - textBlockHeight) / 2;
            } else if (this.verticalAlign === 'bottom') {
                textPos.y = elementHeight - textBlockHeight - this.paddingY;
            } else {
                textPos.y = this.paddingY;
            }
        },

        validate: function() {
            if (!(this.canvas instanceof HTMLCanvasElement)) {
                throw new TypeError('From CanvasTextWrapper(): Element passed as the first parameter is not an instance of HTMLCanvasElement.');
            }
            if (typeof this.text !== 'string') {
                throw new TypeError('From CanvasTextWrapper(): The second, dedicated for the text, parameter must be a string.');
            }
            if (isNaN(this.lineHeight)) {
                throw new TypeError('From CanvasTextWrapper(): Cannot parse font size as an Integer. Check "font" property\'s value.');
            }
            if (this.textAlign !== 'left' && this.textAlign !== 'center' && this.textAlign !== 'right') {
                throw new TypeError('From CanvasTextWrapper(): Unsupported horizontal align value is used. Property "textAlign" can only be set to "left", "center", or "right".');
            }
            if (this.verticalAlign !== 'top' && this.verticalAlign !== 'middle' && this.verticalAlign !== 'bottom') {
                throw new TypeError('From CanvasTextWrapper(): Unsupported vertical align value is used. Property "verticalAlign" can only be set to "top", "middle", or "bottom".');
            }
            if (isNaN(this.paddingX)) {
                throw new TypeError('From CanvasTextWrapper(): Unsupported horizontal padding value is used. Property "paddingX" must be set to a number');
            }
            if (isNaN(this.paddingY)) {
                throw new TypeError('From CanvasTextWrapper(): Unsupported vertical padding value is used. Property "paddingY" must be set to a number.');
            }
            if (typeof this.fitParent !== 'boolean') {
                throw new TypeError('From CanvasTextWrapper(): Property "fitParent" must be set to a Boolean.');
            }
            if (this.lineBreak !== 'auto' && this.lineBreak !== 'word') {
                throw new TypeError('From CanvasTextWrapper(): Unsupported line break value is used. Property "lineBreak" can only be set to "auto", or "word".');
            }
        }
    };
})();